Thickener



Aug. 15, 1939.

C. E. WUENSCH THICKENER Filed May 28, 1936 INVENTOR Y ar/es ir Waerme/7 ATTORNEYS Patented Aug. 15, 1939 yUNITED STATES PATENT oFFicE 12 Claims.

'I'his'invention is concerned with the removal of a portion of the liquid from a mixture of liquid and nely divided solids (pulps). More specifically, an improved thickener. or dewatering device, particularly adapted for removing liquid from pulp containing relatively large amounts of semi-colloidal solids, is contemplated.

The conventional thickener employed in the h eretofore customary practice of ore dressing and allied arts comprises an upright cylindrical tank of relatively large diameter provided with means for feeding in pulp at the top and center, and with a peripheral launder for catching the decant, i. e., the overow of liquid from which solids have settled in whole or in part. The tank is provided with a rotatable vertical central shaft,

to the lower end portion of which is attached two or more cross arms provided with numerous rakes. These rakes are attached to the armsin such a way that they tend to push solids settling in the bottom of the tank along a spiral path to the center.l At this point the settled solids (accompanied by liquid that has not been removed from the upper portion of the thickener) are discharged.

Customarily the vertical rotatable shaft is driven from the top. 'I'his requires a heavy super-structure over the tank for supporting the driving mechanism and means that almost lthe entire length of the shaft is subjected to a power-` ful torque.

In a center feed thickener of this type the sandy material tends to settle near the center of the tank while the slime settles near the periphery. Semi-colloidal material does not settle readily and tends to overflow with the decanted liquid. Hence in order to secure a clear overflow by settling along the horizontal area of the thickener tank must be very great in relation to the volume of pulp treated in unit time. Attempts to increase the feed to such a thickener inevitably result in a turbid liquidoverflow. If the semicolloidal solids which impart the turbidity are valuable, turbidity must be suppressed.

As described in my co-pending application, Serial No. 15,217, led April 8, 1935, I have developed a concentrating process in which a heterogeneous mixture of relatively coarse orefr'agments of different specific 'gravities is separated in a chamber in the presence of a heavy medium consisting of a mixture of nely divided solids such as galena in a liquid such as water. 'I'his medium buoys up the lighter fragments while it permits the heavy fragments to sink. Light and heavy fragments are then separately (Cl. 21o-43)' withdrawn from the separating chamber and washed to free them of entrained medium. The

medium thus becomes diluted and should be i thickened and puried before it is used again.

Attempts to employ the hereinbefore described conventional thickener for this purpose have not been entirely successful. The medium contains large quantities of semi-colloidal galena particles or the like. These particles are valuable and do not settle readily. To insure a complete recovery of such particles, a conventional thickener with a tremendous settling area must be employed.

Moreover, because ofl the heavy character of the thickened pulp which settles from a medium containing large quantities of galena and the like, the power required to rake this pulp to the thickener discharge is considerable. If the shaft is topfdriven, a long length of shaft is subjected "to an extraordinarily high torque.

A further diiliculty encountered in attempts to employ a conventional thickener to dewater the aforementioned heavy medium arose from the fact that the sandy material which settled near the center of the thickener required a rake setting such that the rakes would plow deeply.A and carry a heavy load without becoming jammed. The rakes near the periphery of the thickener were required to handle the slimes which settled in this area, and these rakes required a still different setting. In fact, each rake from center to periphery of the thickener required a different setting for optimum results. Moreover, the character of the solids which were settled changed from time to time, and for best results an almost continuous adjustment of the rakes was called for; Inasmuch as these rakes are submerged, such adjustment was obviously impossible in a conventional thickener.

A further problem encountered in attempts to handle heavy media in a conventional thickener resulted from the necessity of removing froth or scumfrom the top of the pool of pulp. In commercial; operation some contamination of the medium with'oil or other flotation agent is almost inevitable. Consequently, a heavy and persistent froth containing valuable particles of galena or the like collected. This froth must be skimmed from the thickener and broken up so that its mineral content may be recovered.

As a result of my investigations I have de. veloped an improved thickener in which the aforementioned medium may be handled with little difficulty.

Loss of semi-colloidal material along with the separated liquid is eiectively prevented without v underneath the thickener.

relatively short length of shaft is subjected to any appreciable torque. The shaft enters the thickener through a bearing provided with a seal to prevent leakage of pulp.

My improved thickener is provided with rakes which are so fastened to the arms that they automatically adjust themselves to the condition of the material that they are required to move. If the load on any rake increases it is so fastened to the arm that it rides upward and at the same time the top of the rake moves backward slightly so that the rake acquires a new pitch that tends to make itvplow down into the contents of the bottom of the tank instead of merely scraping it. 'I'he rake is thus enabled to adapt itself to the extra load and at the same time the danger of breakage due to overloading is minimized. 1f the load on any rake becomes lessened, this rake is automatically lowered, the lower edge of the rake moves backward slightly and the rake then tends to scrape instead of to plow.

These self-adjustable rakes are of great value when it is necessary to thicken a. product containing valuable or perishable solids, because they tend to remove the solids almost down to the tank bottom and thus avoid the retention of a large quantity of bedding. Thus in the case of galena, the self adjustable rake mechanism prevents the retention of much valuable material in the thickener.

Lastly my improved thickener is provided with a novel vmechanism for removing froth from the for subsequent treatment or use.

` Pulp to be separated isvfed to the center of the thickener through a trough which is supported by-a floating bearing resting on the 4 top of the under-driven vertical shaft. 'Ihis eliminates an expensive superstructure.

VThese and other features of my invention will be understood more thoroughly if reference is made to the following detailed description taken in conjunction with the accompanying drawing, in which Y v Fig. 1 is-a sectional elevation of a thickener of my invention including the under-driven .rotatable shaft, the filter mechanism at the periphery of the thickener, the adjustable rake arrangement, the skimming device, and other features; Y

' Fig. 2 is a perspective representation of the adjustable rake mechanism of my invention;

Fig. 3 is a sectional elevation of the iilter mechanism at the periphery of the thickener showing 2,169,442' the automatic blow-off mechanism to remove Fig. '1 isa detailed sketch of a portion of the filter blow-off mechanism.

'Referring now to Fig. 1, it will .be seen that the thickener comprises a cylindrical tank lof relatively large diameter and having a conical bottom 2. This tank may be constructed of steel,

wood or concrete and the conical bottom should' rest on an adequate foundation. At the center of the conical bottom of the thickener is a well 3 which is provided with a discharge pipe '4. Through this discharge pipe the thickened materialfrom the thickener may flow by gravity or be removed by a pump (not shown).

Fastened to the center of the thickener well and extending into the well is a vertically disposed journal 5 through which a vertical rotatable shaft 6' projects. This shaft is supported on a thrust bearing I below the thickener well.

Between the thrust bearing and the thickener well a gear 8 is rigidly disposed in a horizontal position on the vertical shaft. This gear meshes with a worm 9 which is rigidly attached to a horizontal rotatable shaft III, to which force is applied through a speed reducer or other mechanism (not shown).

The vertical shaft is provided with a lute II adapted to preventthe flow of liquid into the journal 5. This lute is constructed as follows: An annular pan l2 is welded or otherwise rmly attached to the outside of the journal 5. Extending into the annular pan and almost to the bottom thereof isA an .annular cap I3 which is fastened rigidly to the drive shaft. Cap and pan form an annular lute around thebearing which 4is filled with mercury as shown. The depth of the lute should be such that the mercury it contains will not be forced out by the pressure of the pulp within the tank. Mercury is approximately 13 times as heavy as water. Therefore, if 5o the column of pulp exerts a hydrostatic head of `13 feet of water on the lute, a mercury seal more than one foot deep should be provided, i. e., the distance from the upper edge of the pan to the lower edge of the cap should exceed 12 inches. The effective depth of the lute must be calculated from the density and height of the column of pulp which it will be called upon to resist.

Fastened rigidly to the vertical shaft of the thickener a short 'distance above the mercury seal are a pair ofhorizontal cross arms I6 and I1, preferably of I-beam construction and extending almost to the tank walls. Preferably the cross arms are integral with each other. In thickeners of large diameter a wheel I8 is provided 'at the end of the horizontal arms. This wheel rides on a circular track I9 at the periphery of the conical bottom of the thickener tank. The solids which settle at the periphery of the thickenerare in most cases slimy and not suiciently compact to interfere with the movement of the Wheel.

The horizontal arms I6 and I1 are provided with a plurality'of rake mechanisms 20, 2l, 22,`

number of rakes provided is the same as in a conventional thickener, and the rakes on the larm I6 are spaced in echelon with the rakes on the arm I1, so thatthe piles left by the rakes on one arm are encountered by the rakes on the other arm. The details of the individual rake mechanisms are described hereinafter and shown in Fig. 2.

The' vertical shaft extends upwardly to a point above the level of the pulp pool in the tank. At the top of the shaft is a floating bearing 26, which floats or rests upon the shaft but in which the shaft is free to revolve. Attached to the iioating bearing is an end of a feed launder 21 through which material to be thickened or dewatered is fed to the center of the thickener through a chute 28. The opposite end of the feed launder is fastened rigidly above the wall of the tank.

Onecross arm carries two rigid upright rods 29, 30. These rods extend to a point above the normal pulp level in the tank. From the top of the rods hinged drag members 3| and 32 are attached to a fioating launder 33A which is sha1- lower at its outer end (next the tank wall) than at its inner end and is provided with a horizontal scraping lip 33B which projects just out of the surface of the pulp in the thickener. The structure of this floating skimmer is shown in section in Fig. 6. A horizontal float 34 extends along its length. As the cross arm revolves, theskimmer mechanism is dragged around on the surface of the pulp. The froth or scum on the surface falls into the sloping launder and flows by itself or aided by water sprays (not shown) to a spigot 35. This spigot is attached by a flexible connection 36, for example, a piece of hose, to a pump boot 31 fastened to the vertical shaft. A

suction pipe 38 extends into the bottom of the boot. Through this suction pipe by means of a pump (not shown) the scum or froth is removed and sent to any suitable subsequent treatment.

At the periphery of the tank about midway up the side is a peripheral annular compartment 39. An extension 39A of the tank wall projects vertically above the compartment on its inner side. The inner wall of the compartment is made of a perforated plate 49. Over the perforated screen is fastened a piece of cloth or other filter medium 50. The hydrostatic head of the pulp in the upper extension of the tank forces liquid through the filter cloth and the perforated screen into the compartment from whence it may drain through a pipe 40 which projects into the lower portion of the compartment. The compartment is also provided with an air pipe 4|. When the filter cloth has become clogged excessively by the cake built up on its outside surface, the drainage line 40 is closed and the air line 4I is opened. Air then blows through the filter cloth and dislodges the cake which falls to the bottom ofthe thickener and is caught by the outer rakes. is then shut off and the drainage line is opened so that filtration can begin again. The solution is forced through the filter cloth by the hydrostatic head of the column of liquid inside the tank.

The novel rake construction will now be considered. Reference should be made to Fig. 2. Each rake mechanism comprises a plate 43, a hinged arm 44, and an adjustable tie rod 45. The hinged arm is cut at an angle of about 90 at its upper end, and hinged to the lower ange of the I-beam or cross arm I6 or I1. The hinged arm may be made conveniently of a piece of strip steel. The lower end of the hinged arm is cut The air at an angle of 45 or thereabouts, depending upon the average character of the material to be raked. This angle will always be other than 90. This end is hinged to the top edge of the plate near the middle.- Both hinges on the arm should be approximately horizontal. 'I'hrough the center of `the plate approximately perpendicular to its face, the loosely fitting adjustable tie rod passes. The tie rod is threaded at both ends and nuts 46 and 41 hold it in place. The end of the tie rod remote from the blade passes through a hole in a clip 48 which is welded to the lower edge of the I-beam remote from the hinged arm. The blade is trapezoidal in shape. The end of the blade nearest the clip is shorter than the opposite end, so that the blade resting in a normal position on the bedding material in the tank will have its lower edge approximately horizontal.

'I'he purpose and action of the adjustable rake structure will now be explained.

Slimes and sands which settle to the bottom of the thickener have different consistencies. The rake mechanism illustrated in Fig. 2 automatically adjusts itself to the particular condition of the material which is being moved. Thus, the rake mechanisms near the periphery of the tank (which handle slirnes) acquire a different position than the rake mechanisms near the center of the tank where most of the sand is deposited. Moreover, the rake mechanism adjusts itself lo meet varying conditions in the bottom of the tank. For example, if the rake becomes overloaded, the hinges on the hinged arm 44 permit the rake to ride upward. At the same time the tie rod 45 pulls the lower edge of the rake inwardly toward the cross arm so that the rake is tilted at a dierent angle from the vertical and tends to plow more deeply into the material which is being moved. The rake is thus enabled to dig away the obstacle to its progress. When overloading of the rake has thus been overcome,

the rake drops onto a lower surface and flattens out, i. e., the lower edge of the rake moves backward with respect to the upper edge so that the rake functions as a scraper instead of as a plow. Should a heavy accumulation of material occur in the path of the rake a second time, the action is repeated, i. e., the rake rises but acquires a new angle from the vertical so that it bites more deeply into the slime or sands being moved. It will be apparent, therefore, that the rakes along the entire length of the arm adjust themselves to the condition of the'bottom. Thus, the rake mechanisms at the outer end of the arms acquire a pitch appropriate to the movement of slimes and operate principally as Scrapers whereas the rakes near the center acquire a pitch appropriate to the movement of sands and slime together' and operate principally as plows.

The fact that the rakes tend to rise when encountering an obstacle prevents them from becoming overloaded and broken, while the changed pitch of the rakes due to their rise permits them to plow out the obstacle gradually.

The tie rod is adjustable so that the rake plate may be made to stand approximately vertical under normal conditions of raking, and will depart from this position under conditions of overload or underload. f

My adjustable rake mechanisms will also assume optimum raking angles for the continuous removal of slimes which are deposited at the periphery of the thickener in cases where the pulp produces no central sandy accumulations in the thickener.

The next feature of the thickener to be conside'red is the peripheral lter mechanism and the automatic control whereby the filter is blown whenever the cake builds up to a point which excessively impedes the ltration. Now considering Fig. 3, the closed compartment 39 is provided with a screen plate 49 or other backing for the iilter cloth 50. Within the compartment is a baille 5| which is fastened to the top of the compartment and extends downwardly to within a .short distance of the bottom of the compartment.

The drainage pipe 40 projects into the lower portion of the compartment and ends in an upwardly projecting elbow 52. The air line 4I also projects horizontally into the compartment at a higher level and ends in a quick opening throttle valve 53. Thethrottle valve is equipped with a handle 54 which lies ina horizontal position -when the throttle valve is closed. The end of the handle carries a horizontally disposed ring 55 of fairly large diameter. Through this ring 55 projects a, spear-shaped spring 56. A horizontally disposed ring 51 of smaller diameter is disposed below the ring 55 and fastened to the wall of the compartment by a rod 58. 'I'he spear-shaped spring projects through both rings and is fastened to a vertical rod 59 which ends in a flexible ball of rubber or other material 60 above the top of the drainage elbow.

On the rod 59 between the ring 51 and the ball 60 is rigidly fastened two oversized stops 62 and 62A. A lever arm 63 is hinged to the wall of the compartment at one end and is fastened to a rod 64 extending upwardly from a float 65 at the opposite end. The vertical rod 59 passes through a hole in the lever arm 63 between the two oversized stops. The rod 64 projects upwardly through two guides 66 and 61.

A spring 68 is fastened from the handle 54 of the air valve 53 to the top of the compartment and tends to hold the handle in a horizontal position so that the valve is closed.

The spear-shaped spring numbered 56 is made of flat spring steel or other material of like char. acter as shown in detail in Fig. 7. In its expanded condition the barbs on the spear project out far enough to catch on the top of the ring 55 and pull the handle of the air valve downward. However, asl the spear-shaped member is pulled downward, it is compressed in passing through the ring 51. If it is pulled down far enough, it is collapsed so that the distance between the barbs is shortened suiciently to permit it to` pull through the upper ring 55, and release it. If, thereafter, the spear-shaped spring is pushed upward into the ring 55 it will compresssumciently so that the barbs pass .through the ring 55, whereupon the spring snaps back to its original shape, in which the barbs will again catch the ring if the spear is pulled'downward. u

The operation of the mechanism just described is as follows: Y

When liquid is flowing freely through the lter 50 under the baille 5I and out through the drainage line 40, the float is raised high enough to force the lever arm 63 up against the stop 62. This prevents the rubber ball 60 from being sucked down or dropped to close the end of -the drainage elbow 52. However, as the cake builds up on the lter cloth, the flow of solution decreases. The level of liquid in the compartment drops and the floatv 65 is lowered'. 'When it is lowered suiliciently far, the lever arm 63 pulls down the rubber ball and seats itin the top of the drainage elbow. In so doing the barbs of the spear-shaped spring are caught on the upper ring 55 and pull the ring down, opening the air valve 53. When this happens, the remaining liquid in the compartmentis driven down to the lower edge of the baille by air pressure. The air then passes underneath the baille and blows the cake oif the lter. There is a momentary passage of air through the lter. However, by this time the iloat 65 will have dropped so far that the spear-shaped springis collapsed by passing through the ring 51. In this way the barbs are contracted and pass through the ring on the handle of the air valve. The spring 68 then pulls the handle of the air valve -back to a horizontal position. Solution begins to flow through the lter 58 and lls the compartment. As it does so, the iloat 65 rises, the lever arm 63 pushes upwardly against the stop 62 and drives the spearshaped spring up through the ring 5 5. The apparatus is then ready to repeat its action.

The principal virtue of this type of 'control is that the lter is only blown whenever the thickness of cake warrants such blowing. In the heretofore customary types of blowing mechanism, the filter has been blown periodically in time whether or not such blowing was required. In other words, the mechanism of my invention automatically adjusts the blowing cycle to the condition of the filter.

If additional lter area is required, this is easily provided as shown in Fig. 4. One or more additional leaves, the length of which is bent to conform to the diameter of the thickener, are carried inside the thickener adjacent the iilter 50. As shown in Fig. 4, a iilter leaf 10 is. built up from two sheets of vertically disposed perforated screen plates 1I, 12, which are clamped at their lower and upper ends to spacer blocks 14 and 15. Spacer blocks and screen plates are bent in a horizontal direction to form an arc appropriate to their position. If made continuous this structure forms a ring inside the thickener. Surrounding 4these screen plates and the spacer blocks is a nearer the center of the thickener and connected to the compartment 39 by suitable conduits at theirl lower edge. The same blow-off mechanism removes cake from the lter 50 and the lter 10, as well as-from any additional leaves. It is important, however, that the pipe 11 be sufiiciently large that there be a. uniform distribution of air on the surface of the filter 50 and the filter 16.

Otherwise, cake will not be properly removed from the total illter surface.

The advantage of this type of structure lwill be immediately apparent. Thus, for example, the structure shown in Fig. 4 will make available a tremendous filter area even in a thickener with a diameter of, say, fifty feet. For example, if the filter leaves are three feet high, a thickener ilfty feet in diameter constructed according to Fig.4 4 will have approximately 1500 square feet of filter area available on the single leaf 10 and the lter 56.

It is desirable to break up the lter into several segments. In Fig. 5, the peripheral lter 75 .the drain pipe 40.

mechanism is broken into four segments, each provided with separate compartments 80, 8|, 82, 83, into which filtrate drains. Each compartment is provided with its own control mechan'm. By suitable adjustment of the float levels in each compartment, the mechanism may be constructed so that the filters will not all blow E at once. In other words, this type of structure permits cyclic blowing of the various peripheral segments of the ilter. In this way the capacity of the compressor which supplies the blow-oi air may be lessened because a smaller peak load of air is demanded. Moreoverfsome filtrate is always leaving the thickener in this practice and there is less danger of overflow.

For purposes of clarity the operation of the entire mechanism will now be considered. The pulp to be thickened is fed into the center of the thickener through the launder 21 and drops into a pool of pulp contained in the thickener. The amount of pulp fed is such as to keep the tank almost full at all times so that an ample hydrostatic head is available to force liquid through the peripheral filter. The sands in the pulp settle rapidly toward the center of the thickener. The slimes tend to settle further out. Some of the ner slimes do not settle sufficiently to be caught by the rakes. These are eventually caught on the filter 50 and consolidate into a cake. The cake is blown off periodically and falls down where the outer rakes of the thickener may seize it. The filter cake, together with such ne material as has settled in the outer portion of the thickener, is gradually raked toward the well at the center of the thickener bottom. The sand which falls near the center of the thickener is also lraked toward the center. The rake mechanisms adjust themselves to the particular condition at the bottom. For example, if a heavy accumulation of cake falls into the thickener at its outer In this way, the overload is quickly removed but' the rakes are in no danger of becoming overloaded or broken.

The liquid from the thickener runs out through The thickened solids, both sands and slimes, are raked down to the center of the thickener and pass out lthrough the discharge pipe, the paddles I and I5A serving to stir them and prevent clogging of the discharge pipe.

No leakage can occur at the bottom of the thickener because of the submerged mercury seal which is deep enough to sustain the hydrostatic pressure of the height of pulp inthe tank.

Any fro'th which collects on the-surface of the pulp in the thickener is scraped off by the rotating skimmer and flows by itself or is washed into the boot at the center of the thickener. From this point it is withdrawn by the suction pump.

No heavy superstructure for the thickener is required because of the underdrive mechanism. 'I'his substantially reduces the cost of construction.

The adjustable rakes may be used in a thickener which is not equipped with either. the skimming device or the peripheral filter mechanism but instead has a conventional overflow launder (not shown) about its upper edge. Conversely, the filter mechanism can be used in a thickener which is provided with ordinary rakes and without a skimming device,

If the material being Atreated does not form a froth, the skimming mechanism may be eliminated.

The underdrive mechanism may be employed in a thickener which is otherwise of conventional construction.

Although the thickener described herein is particularly adapted to treating heavy media, Vit -is not limited thereto and may be usefully employed in a variety of services.

l. A thickener which comprises an upright cylindrical tank adapted to contain a pool of pulp, a discharge conduit communicating with the lower central portion of the tank, means for introducing pulp into said` tank, a rotatable raking mechanism extending outward from the lower center portion of the tank to a point near the lower periphery of the tank, said raking means being adapted to move solids settled out of the vpulp to the discharge conduit, a filter having substantially vertical surfaces disposed adjacent the wall of the tank below the level of the pulp pool and above the raking mechanism, means for discharging iiltrate from said filter and automatic means for blowing cake oi said filter at intervals determined by the rate of ow of filtrate through said lter.

2. Apparatus according to claim l wherein a compartment provided with means for draining out ltrate and admitting gais `under pressure communicates with the filter.

3. Apparatus according to claim 1 wherein the filter extends substantially around the periphery of the tank.

4. Apparatus according to claim 1 wherein the filter extends substantially around the periphery of the tank and partitions spaced around the periphery are provided for dividing the lter into a plurality of compartments each provided with independent means for blowing oif the cake.

5. In a thickener the combination which comprises an outwardly extending cross arm movable around a verticalaxis, a rake plate, an elongated member fastened at one end by a hinge to the cross arm and at its other end by a hinge to an upper portion of the rake plate, and an adjustable rod fastened loosely to a lower portion of the rake plate and tothe cross arm.

6. Apparatus according to claim 5 in which the rake plate is disposed at an angle with respect to the axis of the cross arm, and the point at -which the rod is fastened to the cross arm is longitudinally remote from the point at which the hinged member is fastened to the cross arm.

'7. Apparatus according to claim 5 in which the hinges are substantially horizontal andthe hinged member is cut square at the cross arm hinge and at an angle other than 90 at the rake plate hinge.

8.` A thickener which comprises an upright cylindrical tank adapted to contain a pool of pulp, a vertical rotatable shaft projecting into said tank through the bottom thereof and extending upwardly to the upper portion of the tnak, said shaft being provided with a thrust bearing on its upper end, a conduit for introducing pulp attached to said bearing, a lute comprising an annular open-toppedpan rigidly disposed around` said shaft and adapted to retain a heavy liquid and a close-topped cap projecting into said annular pan and attached rigidly to the shaft, a discharge conduit communicating with said tank adjacent a central lower portion, one or more outwardly extending arms fastened to the shaft in the lower portion of the tank, and rakes connected to said arms adapted to move solids settled from the pulp toward the discharge conduit.

9. In a thickener comprising an upright cylindrical tank adapted to contain a pool of pulp and provided with an outwardly extending 'rotatable raking mechanism in the lower portion of the tank adapted to move settled solids to a central discharge conduit, the combination which comprises a trough adapted to float on the pool and provided with a horizontal lip above the surface o! the pool, means connecting said trough with the rotatable raking mechanism, and a boot disposed in the central top 'portion of the thickener and communicating with the trough whereby material may be removed from the trough.

l0. In a thickener the combination which comprises an outwardly extending cross arm movable around a substantially upright axis, a rake plate, hinged means connecting an upper portion of the rake plate with the cross arm, and means connecting a lower portion of the rake plate with a point below the point at which the hinged means is connected with the cross arm.

11. In a filter mechanism having'a lter, a filtrate-receiving chamber connected therewith, an outlet conduit for withdrawing illtrate.there from and an inlet conduit for admitting fluid thereto under pressure for blowing cake off the filter, the improvement which comprises a valve on the outlet conduit, a valve on the inlet conduit, a float disposed in said chamber, means for opening the valve on the inlet conduit and closing the valve on the outlet conduit when the oat falls due to a decrease in level of iiltrate in the chamber, and means ior closing the valve on the inlet conduit andl opening the valve on the outlet conduit after the blow-oil fluid has been admitted into the chamber through the valve on the inlet conduit.

12. In a thickener provided with a horizontally disposed settling tank with upright walls adapted to retain a pool of pulpand means for raking setl tled solids across the lower portion of the tank"I from the walls oi the tank to discharge means inv the lower central portion of the tank, the combination which comprises a cylindrical settling tank, a plurality of stationary illters having substantially vertical illter surfaces disposed adjacent and affixed to the cylindrical wall of the tank and below the pulp level therein, means for withdrawing filtrate from said illters, and means for blowing cake oif said lters whereby said cake may fall to the lower portion of said tank and be moved to the discharge means by the raking means.

' CHARLES ERB WUENSCH. 

